Set main attribute in headless package.json back to the dist file

Otherwise the prod webpack build is built in a different way so that:

1. runtime object assignments (e.g. `Object.assign(api, foo)`) aren't
reflected on imported objects (causing undefined attribute errors).
2. circular imports cause crashes, making the build unusable.

Unfortunately fixing these issues on a more fundamental level will
require much more work.

src/headless/package.json

@@ -8,8 +8,7 @@
   ],
   "homepage": "https://conversejs.org",
   "license": "MPL-2.0",
-  "main": "index.js",
-  "browser": "dist/converse-headless.js",
+  "main": "dist/converse-headless.js",
   "module": "index.js",
   "keywords": [
     "converse.js",